The invention relates to a method of purification of a melt of aluminum and its alloys by introduction of an active gas mixture at atmospheric pressure.
One of the usual techniques in the aluminum industry has consisted for a long time in passing gas mixtures through melts of aluminum and its alloys, in order to remove from them dissolved gases, especially hydrogen, as well as metallic impurities, especially traces of alkali metals and alkaline earth metals, and at the same time to free the metal from non-metallic inclusions, namely oxides. One of these methods, generally termed gas treatment or degassing employs generally pure chlorine gas and is expensive, in as much as it requires large quantities of this gas and leads to a loss of aluminum in the form of volatile aluminum chloride. This method brings with it undesired working conditions, because in it corrosive and poisonous waste gases result. For this reason in the state of the art at various times proposals have been made as to how one can economize in chlorine, in that one carries out the degassing by a gas mixture of chlorine and a non-reactive gas, as for example air or nitrogen, or while one carries out the treatment in an atmosphere of dry air. At the same time it has even been proposed to employ nitrogen alone (N.sub. 2) as treatment gas.
Furthermore there has been proposed the use either of carbon dioxide (CO.sub.2) or carbon monoxide (CO) for the treatment of molten magnesium alloys before casting (U.S. Pat. No. 2,380,863). Nevertheless, with aluminum it has been generally assumed that the employment of carbon dioxide would be unsuitable, because this would be reduced partly to free carbon on contact with the aluminum melt, which in turn would react with the aluminum, and produce undesired aluminum carbide, AlC.sub.3, (U.S. Pat. No. 2,369,213). For this reason it was surprising and unexpected that, according to the present invention, a significant improvement of the existing method is achieved, and at the same time the undesired side effects could be extensively reduced, in that a flushing gas was introduced consisting of carbon dioxide and a non-reactive carrier gas, for example an inert gas.
The object of the present invention accordingly consisted in reducing the content of alkali metals and alkaline earth metals, hydrogen and non-metallic inclusions in melts of aluminum and its alloys as far as possible, by a flushing gas treatment, and at the same time to keep the undesired side effects of this flushing gas treatment as slight as possible. In particular the formation of dross and loss of metal with it should be minimized. At the same time the energy costs should be held as low as possible, that is to say the duration of the flushing gas treatment should be minimized and the normal operating temperature of melt treatment of about 770.degree. C. should not be exceeded. Finally, the material costs of the treatment, especially the costs for the flushing gas and possibly for a charge of filter granulate should be minimized and the undesired toxic properties of chlorine gas should be excluded.